This invention relates to automatic lighting devices and, more particularly, to automatic devices for controlling the state of lights in areas depending upon the presence or absence of persons using the lights.
The need to conserve energy which was made apparent to all with the recent oil crisis has precipitated inventions in a great number of areas. One particular area in which invention has occurred has been that related to automatically disabling electrical devices when the use of those devices has been completed. In particular, a number of arrangements have been devised for automatically turning on and off lights so that when an area is not occupied the lights are switched off thereby conserving substantial amounts of energy. Such a device usually utilizes a motion detector to switch the lights on and maintain them in the on condition while persons are present. A timing arrangement is incorporated into the device to provide a period during which movement must be detected or the lights for the area are extinguished. The timing arrangement allows a person to leave the area or to remain inactive for short periods without the lights being extinguished.
Although such devices are quite efficient energy savers, they do cause problems. For example, since such devices automatically switch the lights on whenever a person is present, they automatically activate the light during daytime hours when their use is a waste of energy. Moreover, the detectors used with such systems often activate lights in unused areas when a person simply passes by the area. Areas using automatic lighting devices often cannot be dimmed for displaying slides or movies.
An attempt to solve these problems by utilizing a manual on switch causes additional problems and, under certain circumstances, may render an area unusable for certain activities. For example, in order to be able to sense when an area is empty, the motion detectors used in such devices must be able to ignore various low level movements within the area such as air currents which are not caused by humans; otherwise, the lights would remain on even though no humans remained in the area. Consequently, such devices must be set to ignore some base level of movement.
This threshold level for maintaining the lighting in operation is often not reached even though the area is occupied by a person if the person is involved in some long term mental activity like reading which involves little physical movement. As a result, prior art automatic lighting devices often extinguish the lights while a person is occupied in concentrated mental activity just when that person can least afford the interruption. If a manual switch is used, this necessitates the person moving about to active the lights and forces a break in the person's concentration. If the person again settles into his mental activity for longer than the timed period, the lights are again extinguished. Such a device obviously detracts from the ability of the person to accomplish the particular mental activity.